Testing implants

ABSTRACT

Method and apparatus for testing an implant attached to a bone of a human or animal subject includes a member releasably attached to the implant. The member carries a transducer for exciting the member with a variable frequency AC signal, and a transducer for detecting a resonance frequency of the member. The detected resonance frequency is used to assess the degree of attachment of the implant to the bone.

The present invention relates to a method and apparatus for testing animplant attached to a bone of a human or animal subject. The use ofimplants involves the insertion of a metal fixture into a prepared holein the bone. During the healing process, the surrounding bone developsan intimate contact with the implant surface and after a suitable time aprosthesis may be attached to the fixture. Such implants are frequentlyused in dentistry and in cosmetic surgery.

There is a need for a means of clinically observing the quality of theunion between the bone and the implant surface. Implant failures can becaused by errors in placement, and premature or inappropriate loading. Anondestructive test which could be used before loading the implant wouldhelp to reduce failures of this type, and would also enable periodictests to be carried out on implants which are in use to ensure that theyare still satisfactory. The test could also provide a quantitativecomparison between different implant systems.

X-rays are sometimes used to test the condition of an implant, but theycan only show the presence of gross bone loss around the implant. It isalso very difficult to monitor the progress of integration over timewith x-rays, since it is difficult to reproduce the viewing position andangle with sufficient accuracy. A different sort of test, albeit a crudeone, is to tap the structure attached to the implant with a surgicalinstrument. This test can only distinguish between satisfactory implantsand the most grossly defective systems.

It is therefore an object of the present invention to provide anon-destructive test which is capable of giving a reliable indication ofthe quality and/or extent of the union between an implant and the boneto which it is attached.

Accordingly there is provided a method of testing an implant attached toa bone of a human or animal subject, the method comprising the steps ofbringing a member into contact with the implant; detecting at least oneresonance frequency of the member when it is in contact with theimplant; and interpreting the detected resonance frequency in terms ofthe degree of the attachment of the implant with respect to the bone.

The stiffness of the joint or interface between the implant and thebone, and also the exposed length of the implant, will affect theresonance frequency of the member. Hence, monitoring this resonancefrequency provides a means of assessing the integrity of the joint.

Preferably, the member is releasably attached to the implant.

According to one preferred arrangement, the member comprises acantilever beam. The implant often includes a threaded bore by means ofwhich the prosthesis, or a pillar or post (called an abutment) intendedto carry the prosthesis, is screwed to or into the implant. The abutmentor an associated fixing screw also usually has a threaded bore by meansof which the prosthesis is screwed to or into the abutment. Thecantilever beam, conveniently, can be screwed to or into the implant, orabutment, using the associated threaded bore in the latter.

The detected resonance frequency is conveniently compared with one ormore values for the resonance frequencies of the same or similar membersin contact with other implants. By comparing the detected resonancefrequency with values obtained on other satisfactory or lesssatisfactory implants, an indication of the degree of integration of theimplant can be obtained. Furthermore, the same implant could be testedwhen it is initially inserted, and periodically thereafter, both duringthe healing process, when it is intended to attach the prosthesis, andthereafter, and the various resonance frequency values compared, toobtain an indication of the progress of the integration process, whetherand when a prosthesis or abutment should be attached, and, subsequently,whether the condition of the implant is still satisfactory.

The resonance frequency is conveniently detected by exciting the memberwith an AC signal, detecting the response of the member to the ACsignal, and varying the frequency of the AC signal until the detectedresponse of the member is a maximum. Other methods of detecting theresonance frequency are equally practicable.

The invention further resides in apparatus for testing an implantattached to a bone of a human or animal subject, the apparatuscomprising a member adapted to be releasably attached to the implant;and means for detecting at least one resonance frequency of the memberwhen it is attached to the implant.

The apparatus conveniently includes means for exciting the member withan AC signal, and a transducer for detecting the response of the memberto the AC signal, the arrangement being such that the frequency of theAC signal is varied, and the transducer detects when the response of themember is at a maximum. The transducer preferably comprises apiezoelectric element, and the means for exciting the member may alsoconveniently comprise a piezoelectric element driven by a variablefrequency oscillator. The detection and/or excitation means couldalternatively comprise magnetostrictive or electromagnetic devices.

The invention will now be further described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of one embodiment of apparatus accordingto the invention;

FIG. 2 is a graphical representation of a typical frequency responsecurve of a cantilever beam attached to a typical implant;

FIG. 3 is a graphical representation of the hypothetical change in theresonance frequency, over a period of time, of a cantilever beamattached to a typical implant; and

FIG. 4 is a schematic view of a second embodiment of cantilever beam.

Referring to FIG. 1, the apparatus comprises a member in the form of acantilever beam 1 attached by means of a threaded section 2 to animplanted fixture, such as a dental implant 3, in a section of bone 4,typically a human jaw bone. The implant 3 may be any one of a number ofknown types, formed from a metal, such as titanium, from a ceramicmaterial, or any other appropriate material. It may, for example, be ofthe type supplied by Nobelpharma in the U.K. Two transducers, such aspiezoelectric elements or strain gauges 5 and 6, are attached, forexample bonded, to opposite sides of the beam 1, gauge 5 being anexciter gauge and gauge 6 a receiver gauge.

The exciter gauge 5 is driven by a variable frequency oscillator,signals from which, for example in the form of a sinusoidal excitationvoltage, are fed to the gauge 5 via an amplifier. The oscillator andamplifier may be incorporated in a frequency response analyser 7.Signals detected by the receiver gauge 6 are amplified by a chargeamplifier 8 and applied as an input to the analyser 7. The output fromthe analyser, which represents the ratio of the response voltage to theexcitation voltage, is fed to a processor such as a microprocessor 9,which is used to vary the frequency output of the oscillator of theanalyser 7, and store the results in a data store 9a. The results may beprinted out, and/or displayed on an oscilloscope 10, and/or an ACvoltmeter or the like.

In use the beam 1 is secured, i.e. screwed, to the implanted implant 3with a predetermined torque, for example using a Nobelpharma torquecontroller and counter tool. The variations in resonance frequency withtorque have been found to be relatively small over a practical range oftorques, for example of the order of 10 to 15 Ncm, so that such torquevariations should not present a problem. Constant amplitude, for example1 volt, AC excitation signals are then applied to the beam 1 via thegauge 5. The frequency of the AC excitation signals is varied until theamplitude of the signal displayed on the oscilloscope 10 is at amaximum. The resonance frequency is the frequency at which the amplitudeof the ratio of the response voltage to the excitation voltage is amaximum. FIG. 2 shows the data from a coarse sweep which is used toobtain the resonance frequency roughly. A finer sweep around this regionis then used to identify this frequency, typically the first orfundamental frequency, more accurately. This frequency is noted, andcompared, for example, with the data for other implants at similarstages of bonding.

It is expected that for a particular implant, the resonance frequencywill vary with time as depicted in FIG. 3. Thus by comparing thedetected resonance frequency with previously compiled data for similarimplants, an indication of the degree of attachment of the implant canbe obtained. With regard to FIG. 3, the stiffness of the interface mayinitially decrease following implant placement because of acuteinflammatory response. The stiffness then recovers as integrationoccurs, and is expected eventually to approach, reach or exceed theinitial value.

The technique, which is based on detection and comparison of resonancefrequency shifts, rather than amplitude changes, is effective todetermine the quality of the implant/tissue interface as a function ofits stiffness, and also in relation to any bone loss as a function ofthe level or height of the marginal bone surrounding the implant.

A currently preferred cantilever beam is illustrated in FIG. 4. Thisbeam 1 is generally L-shaped, having base limb 1a with an aperture 1bwhich locates over a boss 3a at the upper end of the implant 3. The beamis fixed in place by a screw 11 screwed into the threaded bore in theimplant. The aperture 1b and boss 3a may be non-circular, for examplehexagonal in cross-section, so that the beam orientation about thelongitudinal axis of the implant may be accurately and repeatedlydetermined. Different readings may be obtained for different angularorientations of the beam relative to the implant, so as to determine thestiffness/bone level at different positions around the implant axis.

The beam 1 as shown in FIG. 1 or 4, which will preferably be of the samematerial as the implant, for example titanium, is dimensioned so as toprovide a resonant frequency range of the system (placed implant andbeam) of the order of 1 to 20 kH, more specifically 5 to 15 kH, andpreferably in the region of about 10 kH. For example, in the embodimentof FIG. 4, the limbs of the beam 1 may be of approximately 5 to 6 mmsquare crosssection, the upright limb being approximately 2 cm high, andthe base limb being approximately 1.5 cm long.

It will be understood that various modifications may be made withoutdeparting from the scope of the present invention as defined in theappended claims.

For example, an additional pair of excitation/detection transducers orgauges may be mounted on the sides of the beam at 90° to the transducersor gauges 5 and 6 shown, so as to provide readings at right angles tothe latter transducers, without the necessity of re-orienting the beamon the implant. Additionally, or alternatively, the beam and/ortransducer system could be adapted to turn relative to the implant.

Although the beam shown in FIG. 4 is L-shaped, the upright limb couldform a straight extension of the base limb 1a so as to lie generallyparallel to the jaw or mandible.

In practice, the prosthesis may be attached directly to the implant 3using the threaded bore in the latter. Alternatively, the prosthesis maybe indirectly attached to the implant via a separate pillar or post(called an abutment). Such an abutment has means, such as an axial screwpassing completely through the abutment, which threads into the implantbore, to fix the abutment to the implant. The upper end of the screw, orthe abutment, has a threaded bore for attaching the prosthesis. The beam1 may be attached, in the manner previously described, to the upper endof the abutment. The beam may then be employed, not only to assess theintegrity of the implant/bone interface, but also the integrity ofabutment/implant joint.

The transducers or gauges, and optionally also the beam may be coated,for example with an air dry acrylic material, to protect the transducersduring sterilization of the apparatus. The electrical connections orwires connected to the transducers are arranged or adapted to minimizetheir damping effect on the resonant structure. The member may take aform other than a cantilever beam, and/or the piezoelectric transducerscould be replaced by other receiver/transmitter elements, for exampleemploying sonic resonance. The beam, instead of being basicallystraight, could be generally U-shaped, and connected to the implant orabutment by its base. The transducers or equivalent could be mounted onthe same or opposite limbs.

We claim:
 1. A method of testing an implant attached to a bone of a livesubject, the method comprising the steps of bringing a member intocontact with the implant; detecting at least one resonance frequency ofthe member when it is in contact with the implant; and interpreting thedetected resonance frequency in terms of the degree of attachment of theimplant with respect to the bone.
 2. A method according to claim 1,including the step of releasably attaching the member to the implant. 3.A method according to claim 1 or 2, wherein the member comprises acantilever beam.
 4. A method of testing an implant attached to a bone ofa live subject, the method comprising the steps of releasably attachinga cantilever beam to the implant; detecting at least one resonancefrequency of the beam when it is attached to the implant; andinterpreting the detected resonance frequency in terms of the degree ofattachment of the implant with respect to the bone.
 5. A methodaccording to claim 4, wherein the implant includes a threaded bore, andthe member is a cantilever beam secured to the implant.
 6. A method oftesting an implant attached to a bone of a live subject, the methodcomprising the steps of releasably attaching a member to the implant;detecting at least one resonance frequency of the member when it isattached to the implant; and interpreting the detected resonancefrequency in terms of the degree of attachment of the implant withrespect to the bone by comparing the detected resonance frequency withat least one value for the resonance frequency of an equivalent memberin contact with another implant.
 7. A method of testing an implantattached to a bone of a live subject, the method comprising the steps ofreleasably attaching a member to the implant; detecting at least oneresonance frequency of the member when it is attached to the implant;and interpreting the detected resonance frequency in terms of the degreeof attachment of the implant with respect to the bone by comparing thedetected resonance frequency with at least one other value, taken at adifferent time, for the resonance frequencies of an equivalent member incontact with the same implant.
 8. A method of testing an implantattached to a bone of a live subject, the method comprising the steps ofreleasably attaching a member to the implant; exciting the member withan AC excitation signal; detecting the response of the member of the ACexcitation signal; varying the frequency of the AC excitation signaluntil the detected response of the member is at a maximum, therebydetecting at least one resonance frequency of the member when it isattached to the implant; and interpreting the detected resonancefrequency in terms of the degree of attachment of the implant withrespect to the bone.
 9. A method according to claim 8, includingderiving an output which is the ratio of the voltage of the ACexcitation signal to the voltage of a response signal corresponding tothe response of the member to the AC excitation signal.
 10. Apparatusfor testing an implant attached to a bone of a live subject, theapparatus comprising a member adapted to be releasably attached to theimplant; and means for detecting at least one resonance frequency of themember when it is attached to the implant attached to said bone. 11.Apparatus according to claim 10, wherein the member comprises acantilever beam.
 12. Apparatus for testing an implant attached to a boneof a live subject, the apparatus comprising a member adapted to bereleasably attached to the implant; excitation means for exciting themember with a variable frequency AC excitation signal; and meansincluding a transducer for detecting at least one resonance frequency ofthe member when it is attached to the implant attached to said bone andexcited by the AC excitation signal, by detecting the response of themember to different frequencies of the AC excitation signal, todetermine when the response of the member is at a maximum.
 13. Apparatusaccording to claim 12, wherein at least one of the excitation means anddetector means comprises a piezoelectric element, the excitation meansbeing driven by a variable frequency oscillator.
 14. Apparatus fortesting an implant attached to a bone of a live subject, the apparatuscomprising a cantilever beam adapted to be releasably attached to theimplant; and means for detecting at least one resonance frequency of thebeam when it is attached to the implant attached to said bone. 15.Apparatus according to claim 14, wherein the beam is a generallyL-shaped beam having a base limb, means being provided to rigidly attachthe base limb of the beam to an implant.
 16. Apparatus as claimed inclaim 14, wherein the beam is adapted to resonate at a frequency withinthe range of about 1 to 20 kH.
 17. Apparatus as claimed in claim 14,wherein the beam is adapted to resonate within a frequency range ofabout 5 to 15 kH.
 18. Apparatus as claimed in claim 14, wherein the beamis adapted to resonate at a frequency of the order of 10 kH.